1. Field of the Invention
The invention relates to optical devices. In particular, the invention relates to optical attenuators.
2. Background of the Invention
A variety of optical devices employ optical attenuators to reduce the intensity of light signals carried within the optical device. These optical devices can employ ridge waveguides to confine light signals to particular regions of the optical device. A ridge waveguide is generally defined by a ridge extending from a slab of light transmitting medium.
Attenuators used in conjunction with ridge waveguides often employ free carrier absorption to achieve attenuation. These attenuators typically include doped regions positioned on opposing sides of a ridge waveguide. Electrical energy is applied to the doped region to inject free carriers into the waveguide.
These attenuators can employ trenches extending into the slab region on opposing sides of a ridge waveguide. The doped regions extend from the bottom of the trenches to the base. This structure serves to constrain the injected carriers between the doped regions and accordingly increases the efficiency of the attenuator.
The trenches can interact with light signals traveling along a ridge waveguide. For instance, the trenches can reflect or scatter a light signal entering the region between trenches. Further, the trenches can make the waveguide a multimode waveguide that excites higher order modes. The higher order modes can interfere with the fundamental mode and serve as a source of wavelength dependent loss. As a result, the trenches can be a source of optical loss and a source of wavelength dependent loss. There is a need for a high efficiency attenuator associated with reduced optical loss when operated in the pass mode.